Accurate and quantitative clinical assessment of bileaflet mechanical valves has remained an elusive goal for cardiologists due to a paucity of information regarding the various hemodynamic factors which influence effective valve area measurements and due to the complex flow patterns through the valve orifices, which create difficulties when using current methods to assess forward effective orifice area such as the Gorlin & Gorlin formula and Doppler continuity equation. Despite this, bileaflet valves are one of the most popular designs implanted in pediatric and adult patients, creating an urgent need for increased quantitative information about the hemodynamic influences affecting chronically implanted valves. We propose to apply fundamental hydrodynamic principles to analyze existing digital color Doppler data of flows through bileaflet valves (St. Jude: sizes 23-29 mm) that had been chronically implanted in the mitral position in sheep. During the experiments, color Doppler data were obtained under various hemodynamic conditions including changes in pressure gradient, stroke volume and heart rate. Flows through each of the three orifices will be calculated using a modified proximal flow convergence equation specifically derived from in vitro measurements of the actual velocities through the valves. Calculated flows, cross-checked against simultaneously obtained electromagnetic flow meter data, will be used to compute effective orifice area for each of the three orifices by dividing flow by the continuous wave Doppler velocity time integral through the orifice. Effective orifice areas will be calculated using the modified proximal flow convergence technique, the Doppler continuity equation with known stroke volumes. and the Gorlin and Gorlin formula using catheter measured pressure gradients. Effective orifice areas calculated from each technique will be compared to each other and to anatomic area measured after explanation using direct video planimetry. Effective orifice areas will also be analyzed as functions of heart rate, cardiac output, pressure gradients, stroke volumes and valve size. Our results will have direct clinical relevance to Doppler studies of bileaflet mechanical valves implanted in the mitral position in patients. Furthermore, quantitative information on the functioning of chronically implanted "normal" prostheses will directly benefit surgeons and cardiologists.